The invention relates to an apparatus for measuring the vertical average flow velocity and the depth of water to measure the flow quantity of an open channel, e.g., a river. The invention also relates to a measuring apparatus for measuring the depth of water, water velocity and water temperature of a lake, dam, reservoir, river and the like.
As a typical method for measuring the flow quantity of a point of an open channel such as river, it is know that a cross section of the point and the flow quantity of the point are to be measured. It's specific method is as follows. First, an open channel is divided into a number of sections along a imaginary line drawn along the width of the channel. Then, in each section, the flow velocities are measured at various vertical depths along the vertical center line thereof using a propeller flow-meter, e.g. Based on this, the average flow quantity of each section is calculated. Then, the flow quantity of each point is calculated by multiplying the average flow velocity by the cross section. Finally, the open channel's flow quantity is obtained by summing the all flow quantities of the points.
This conventional method is disclosed in Japanese patent provisional publication No. 9-196727, entitled "Apparatus and method for measuring the river flow quantity", and Japanese patent application No. 9-340875, entitled "Apparatus for measuring the flow velocity" which is not published yet.
The accuracy in the conventional method for measuring the flow quantity becomes higher when many vertical lines are provided to divide the cross section of the water. The vertical lines should be prepared at least 10. In order to measure the average vertical flow velocity along a vertical line, a local flow velocity should be measured at various depths utilizing a local flow velocity gauge (for instance, a propeller-type gauge). The average vertical flow velocity is calculated by substituting the values obtained by the measurement into a formula. When an accuracy in a measurement is required, each vertical line should be divided into 5 to 10, and the local flow velocity is to be measured at each point.
However, since the flow velocity roughly varies at each local point, it takes more than 60 seconds in a measurement operation in each point. When there are provided 10 vertical lines , and the flow velocity is measured at 5 points on each vertical line, it requires more than 3000 seconds to complete the measurement operation. Further, considering the time period needed for moving a measuring apparatus or a gauge, a lot of time is required for a measurement of the flow quantity. Besides, a lot of manpower is needed.
To ease the heavy work needed in the conventional method, there is a permanent river flow quantity measurement post, which automatically measures the local flow velocities along the vertical lines by moving a local flow measuring gauge with, for example, a carrier. However, this method also requires a lot of time to complete the measurement. This problem remain unchanged. Further, in case the flow quantity varies shortly, the obtained flow quantity which has been measured a while ago differ from the flow quantity flowing right now.
In order to solve such drawback, there is a consideration of reducing a number in the local flow velocity measuring points or reducing a time period used in measuring the flow velocity at each local point. However, in doing so, the errors in the local flow quantity and the flow variation are generated. Subsequently, the error in the flow quantity measurement becomes larger.
In order to solve the drawback in the conventional method, a new apparatus for measuring the flow velocity has been developed. The apparatus basis a principle that the propagation velocity of an ultrasonic wave and the frequency of the reflected wave vary in water depending on the flow velocity. This type of apparatus for measuring the flow velocity is superior to the aforementioned mechanical-type measuring method which has commonly been used. That is, it does not disturb the flow velocity of an open channel; it is stable in measurement through the dead flow velocity to the high flow velocity, so that a line showing the measured velocities in a graph is linear; it can measure the directional element of the flow velocity; it can be used in real time measurement; it performs continuous automatic measurement; it is easily maintained since it includes no parts which are mechanically operated.
The method for measuring the flow velocity using the ultrasonic wave includes a propagation time difference method, a phase difference method, a sing around method, Doppler effect method and a beam displacement method. Among these methods, an apparatus using the propagation time difference method is disclosed in the aforementioned Japanese patent provisional publication No. 9-196727 and Japanese patent application No. 9-340875. The apparatus measures the vertical average flow velocity from the water bed to the water surface of an open channel by use of the ultrasonic wave.
The method for measuring the flow velocity utilizing the propagation time difference will be described below referring to an apparatus disclosed in Japanese provisional publication No. 9-196727. As illustrated in FIG. 5, a pair of transducers 1, 1' for measuring the flow velocity is positioned just below the water surface as it is fixed to a catamaran float 4 which is floating on the water surface. Each transducer is positioned at an equal distance D from the center of the float 4 in the same level. A transducer 2 for measuring the water depth is positioned just below the water surface along the center of the catamaran float 4. Another transducer 2' for measuring the water depth is positioned below the transducer 2 with a vertical distance l. Further, an ultrasonic reflecting device 3 is positioned on the river bed as needed.
In FIG. 5, the distance L is a space between the transducer 2 for measuring the water depth and the upper surface of the ultrasonic reflecting device 3. When a distance between the surface of the river and the transducer 2 is a, the water depth H=L+(a+b). The vertical distance l is arranged to a length which is less than 1/2 of the water depth H.
The propagation time periods t.sub.2 and t.sub.2' are calculated by the equations (a) as shown below. The propagation time period t.sub.2 is a time period between the time an ultrasonic wave is transmitted from the transducer 2 and the time it returns to the transducer 2 after reflecting at the reflecting device 3. The propagation time period t.sub.2' is a time period between the time an ultrasonic wave is transmitted from the transducer 2' and the time it returns to the transducer 2' after reflecting at the reflecting device 3. ##EQU1##
In explaining the same point of the prior art disclosed in Japanese provisional publication No. 9-196727 in a different expression, when the ultrasonic velocity measured at a point on the vertical distance l is Cl, the ultrasonic velocity Cl is obtained by equation (b). And, when the total average ultrasonic velocity in the distance L is CL, the distance L is obtained by equation (c). Therefore, supposing the distance L is a distance L', the distance L is obtained by equation (d) which is formulated by substituting the equation (c) into the equation (b). (The rightmost side formula in the equation (d) is the same expression as that described in the above-mentioned prior invention.) ##EQU2##
Further, the time difference .DELTA.t is measured, which is a time difference between a first time period and a second time period. The first time period is a time in that an ultrasonic wave is transmitted from the transducer 1 and is received by the transducer 1' after reflecting at the reflecting device 3. The second time period is a time in that an ultrasonic wave is transmitted from the transducer 1' and is received by the transducer 1 after reflecting at the reflecting device 3.
A general formula for calculating the vertical average flow velocity is shown in equation (e). In this equation, C is the average ultrasonic velocity of the instant velocities transmitted in the ultrasonic propagation route, so that CL should be used as C. However, supposing the vertical average flow velocity is V.sub..perp.' when Cl=CL, the vertical average flow velocity V.sup..perp.' between the river bed (or near the river bed) and the river surface (or near the river surface) is calculated by equation (f), which is formulated by substituting the equation (b) into the equation (e). Numeral reference ".perp." in the vertical average velocity V.sub..perp. denotes the "vertical" of the vertical average velocity. Reference numeral D is a direct distance between the transducers 1 and 1'. The rightmost side of the equation (f) is similar to the equation disclosed in the aforementioned prior invention. ##EQU3##
However, when the water depth and the flow velocity are measured in accordance with the prior art, errors in measurement become significant. That is, in the prior art, as clearly shown in the equations (d) and (f), the length and the flow velocity of the distance L are obtained based on the local ultrasonic velocity cl measured along the vertical distance l, and they are not obtained based on the total average ultrasonic velocity CL in the distance L. That is, only the ultrasonic velocity Cl between the transducers 2, 2' is subjected for the measurement.
As well known, the temperature of a river varies depending on its depth. In summer, the temperature of the surface of a river is high, and it gradually decreases toward the river bed. In winter, on the other hand, the temperature around the surface of the river tends to be higher than the river bottom. Therefore, when merely the substantial surface of a river is subjected to the measurement of the ultrasonic velocity cl, errors in measurements become significantly large since the temperature in a river varies depending on the depth. The length of the distance L measured based on the ultrasonic velocity cl also incurs a large error, and similarly, the measurements in the flow velocity and the flow quantity incur large errors.
In order to show an error incurred in the measurement of the ultrasonic velocity caused by the temperature variation of a river will be described below. Here, a river in summer time is chosen. As illustrated in FIG. 6, the water depth H between the surface and the river bed of the river is equally divided into 3 sections. Suppose, the distribution of the river temperature directly varies in each section.
The river temperature changes 24.degree. C. to 22.degree. C. in the first section which includes the river surface, 22.degree. C. to 18.degree. C. in the second section which is the middle of the river, and 18.degree. C. to 15.degree. C. in the third section which locates at the bottom of the river. To simplify the explanation, the water depth H is supposed to be the same as the distance L, and the vertical distance l is supposed to be 0.2 m. The ultrasonic velocity varies only by the river temperature. The ultrasonic velocity is measured by equation (g) which is a regular relational equation. (Numeral reference T is the average temperature in a section in which an ultrasonic wave propagates.)
When, the water depth H is 3, 4, 5, and 10 m, the local average ultrasonic velocity c.sub.l in the vertical distance l located nearby the river surface is calculated by equation (g). The results are shown in table 1. The total average ultrasonic velocity C.sub.L in the distance L is calculated as 1485.1066 m/s, regardless the water depth. In the prior invention, only the local ultrasonic velocity c.sub.l in the vertical distance l is measured, instead of the total average ultrasonic velocity C.sub.L in the distance L, and the result of the measurement is assumed to be the same as the total average ultrasonic velocity C.sub.L. Therefore, the margin of error .delta..sub.c of the total average ultrasonic velocity C.sub.l against C.sub.L, which is calculated by equation (h), becomes that as shown in table 1. The margin of error .delta..sub.L included in the distance L, which is calculated by equation (d), and the absolute value .DELTA.L are also shown in table 1. ##EQU4##
TABLE 1 (1 = 0.2 m, c.sub.L = 1485.1055 m/s) L( m) 3 4 5 10 C.sub.l (m/s) 1495.3979 1495.5205 1495.5941 1495.6431 .delta..sub.c (%) +0.693 +0.701 +0.706 +0.710 .delta..sub.L (%) +0.693 +0.701 +0.706 +0.710 .DELTA. L (cm) +2.08 +2.80 +3.53 +7.10
Referring to table 1, in case the depth of the water is 5 m, the margin of error .delta.L, that is the margin of error in the depth H, becomes 0.706% even errors in the time periods t2, t2' and the distance l are supposed to be 0. Although this numeral value does not seem to be large, the absolute value .DELTA.L which is calculated by this value becomes 3.53 cm. In the field of the hydrologic measurement in which the prior invention is performed, the worldwide maximum allowable error is .+-.1 cm, thus, the error incurred in this prior invention is not acceptable.
The margin of error .delta.V in the flow velocity measurement which is calculated by equation (f) becomes twice larger than that of the ultrasonic velocity error .delta. c since it is calculated by applying the ultrasonic velocity Cl1 into the equation (f). That is, the margin .delta.V.times.2.times.0.706=1.41%. Further, the local flow quantity q is calculated based on the vertical average flow velocity V.perp. and the depth of water H, so that the error in the local flow quantity q incurred caused merely by the measuring method increases to about 2.1% (.delta.L+.delta.V.apprxeq.2. 1%).
In an actual error in the depth measurement .delta.L, the errors in the time periods t2, t2' are added, so that it exceeds 0.706%. Similarly, the error in the flow velocity measurement becomes more than 1.41% as the errors in such as D, .DELTA.t are added. As a result, the local flow quantity error exceeds 2.1%. This is the biggest drawback in the prior invention.
In many of the hydrologic measuring posts, there is a measuring method that a local flow velocity and a weight is fixed to a rope, and they are dropped into a river from a bridge through use of a gypsy winch. When the measurement point is as high as 5 to 6 m from the river bed, there is another method in which a flow gauge is fixed to a rod instead of a rope, and the gauge is placed in a river to measure the flow velocity and the flow quantity.
In many measuring posts where no such carriers are equipped, a local flow velocity gauge is carried to measure the flow quantity. However, the prior invention disclosed in Japanese provisional publication No. 9-196727 is difficult to carry because of its size and structure. Further, there is a problem that a carrier is needed in order to carry the apparatus to a measuring point, so that it can only be used in permanent measuring posts.
In hydrologic measuring posts, the water temperature has also been measured as well as the depth and the flow velocity for calculation of the flow quantity. As the environmental problem has been a critical issue in recent years, the measurements of water pollution including the water temperature have widely been performed in especially lakes and reservoirs. Therefore, there is a requirement of providing a measuring apparatus for the water temperature.
Regarding to the measurement of the water temperature, a reversing thermometer has been used, which comprises mercury or alcohol and is read after it is pulled out of the water in each measurement. There is also a thermometer comprising a platinum resistance or a thermistor by which the real-time water temperature can be read on the ground. However, each of the thermometers is only capable of measuring the water temperature at a small point where the sensor of the thermometer takes place. Therefore, it is necessary to measure at many points along the vertical direction in the water, and it required a lot of labor and time for the operation. Besides, they lack the accuracy in measurements.
Further, the sensors used in the conventional thermometers have time constant (delay of response) caused by its principle (in the type comprising mercury and alcohol) or its structure (in the type comprising platinum resistance or thermistor), so that it was necessary to prolong the measuring time period and that it brought measurement errors when the measuring time period was shortened. Besides, it was impossible for the conventional thermometers to perform an accurate measurement where the water temperature varies shortly since they required many measurements at many points and they had the time constant.
Therefore, the object of the present invention is to provide an apparatus that is capable of measuring the depth of water, the flow velocity, the water temperature and the flow quantity of an open channel based on an ultrasonic velocity, and that is used at any place, easy to carry, simple in structure and easy to use.